Carbon dioxide insertion into metal ligand bonds

In recent years there has been a growing interest in the chemistry of carbon dioxide, stimulated by current anxieties about alternative petrochemical feedstocks. One aspect under active exploration involves carbon dioxide activation via coordination to a transition metal, and indeed transition metal ions do form C02 complexes.177 The number of simple and reasonably stable complexes is still relatively small and usually limited to low oxidation state metal ions. There are many systems where C02 is used as a reagent giving rise to systems which, while not true C02 complexes, may simplistically be viewed as the products of insertion into metal-ligand bonds, e.g. reaction (9), where if L = H, formates are produced if L = OH, carbonates or bicarbonates result L = NR2 yields dialkylcarbamates and if L = R, carboxylate products result. Much of this area has recently been reviewed and will not be considered further.149... 

Ligand addition to M2 (OR)e is observed as illustrated by - Mo2 OCH2CMe3)6L2 (L = NMes, PMe2Ph), and W2(OPr-i)6(Py)2. Carbon dioxide inserts into metal-amide and metal-alkoxo bonds with retention of the multiple metal-metal bond ... 

The low reactivity of many transition metal alkyls toward CO2 insertion may be attributable to the less polar nature of the M-R bond or to steric hindrance from ancillary ligands. The poor coordinating ability of CO2 may also be a factor via-a-vis CO. Carbon monoxide insertions into metal-alkyl bonds are common for transition metal systems. Knowledge about CO2 insertion into metal alkyls may lead to catalytic systems for carboxylation of organic substrates, an initial step for producing organic compounds from carbon dioxide. 

Buhro WE, Chisholm MH, Folting K, Huffmann JC (1989) Phosphinecarboxylate ligands formed by insertion of carbon dioxide into metal-phosphido bonds. Preparation and structural characterization of tetrakis(di-/er/-butylphosphinecarboxylato)dimolybdenum. Inorg Chem 26 3087-3088... 

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

The possibility of using C02 for the synthesis of fine chemicals that are now derived from petroleum has prompted efforts to obtain a broader understanding of the coordination chemistry of CO2 during the past 20 years.1-21 Carbon dioxide utilization will inevitably center on metal complexes and their ability to bind C02. In the past decade, many C02—metal complexes have been prepared and the ligand has demonstrated a remarkable variety of coordination modes in its complexes. The sections below outline the synthesis, characterization by X-ray crystallography and IR spectroscopy, and some characteristic reactions of these compounds. Also discussed are C02 insertion reactions into M—X bonds and oxidative coupling reactions between C02 and unsaturated substrates which occur at some metal centers. Finally, a profile of the research on catalytic reductions of C02 is provided. Where possible, references are made to reviews rather than to the primary literature. 

Insertion of sulfur dioxide (SO2) into the metal-carbon bond of transition metal alkyl and aryl complexes has also been studied extensively. SO2 shows several binding modes to transition metals as shown in Scheme 7.15 because it is amphoteric, behaving as a Lewis acid and a Lewis base. The Lewis base character of SO2 provides the structural types r/ -planar (3) or (S,0) (4) where SO2 donates a pair of electrons to the metal accompanied by rr back-bonding from filled d orbitals of the metal atom. The Lewis acid behavior of SO2 as a ligand affords an 17 -pyramidal bonding mode (5) where SO2 accepts a pair of electrons from the metal. As ligands tike olefins or carbon dioxide generally tend to prefer... 

Numerous examples of insertion of carbon dioxide into metal to carbon bonds are known. This reaction usually affords ester products. However, treatment of liganded diphenyltita-nium gives rise to the formation of the mono- 98 and the diinsertion product 99 . 

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